mfd: ab8500-core: Ignore masked out interrupts

AB8500 asserts LATCH bits for masked out interrupts.  This patch
explicitly masks those out using the cached mask value to prevent
handle_nested_irq() being called for masked IRQ on the same register as
unmasked ones.

Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Fabio Baltieri <fabio.baltieri@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>